The word "Esky" is in fact a brand name produced by the company Nylex. The term originates from the word "eskimo" obviously for its cold connotations.
Its popularity has become so great during the years that the brand-name has now been used to describe all types of portable coolers whether made by the Esky Brand or not.
They come in many shapes, sizes and colours, but the most well known kind is normally used to store beers and ice. Every good aussie will have a nice large Esky ready for that next bbq occasion.